1 Technical Field of the Invention
The present invention relates generally to a torque detecting apparatus for an AC motor, and more particularly to an improved circuit structure of a torque detecting apparatus for measuring torque of an AC motor based on phase current flowing through each winding of the motor.
2 Background Art
Such a type of torque detecting apparatus is conventionally designed to detect a current flowing through each winding of an AC motor and output the greatest one of the currents detected as indicating a torque value of the AC motor.
In general, the current flowing through each winding of an AC motor varies in sine wave form according to motor rotation. For example, a three-phase motor has a given phase angle of 120 deg. in electrical angle, while a two-phase motor has a given phase angle of 180 deg. in electrical angle. Thus, a conventional torque detecting apparatus measures torque of the AC motor by selecting based on a motor angular position a-winding through which a maximum current flows, to detect the current of the selected winding as indicating a torque value.
Such a prior art torque detecting apparatus, however, encounters the drawback in that a variation component of current flowing through each winding will be contained as ripple in a torque value measured since the current of each winding varies in sine wave form.
Accordingly, when a deviation between measured motor torque and target torque is determined to correct energization current flowing through each winding so as to maintain a constant value of zero (0) under feedback control, a control signal provided based on the deviation will undesirably include ripple components, thus leading to unstable feedback control.